There are two approaches known for synthesis of self-healing polymer composite material, extrinsic self-healing and intrinsic self-healing.
In extrinsic self-healing, polymer composites include a polymer matrix containing sets of capsules or hollow fibers filled with healing agents and catalyst. Any damage or cracking of a polymer composite ruptures the capsules or hollow fibers that yield the healing agents, which fill the cracks via capillary forces. Subsequently, the healing agents undergo polymerization reaction due to catalyst present in the polymer composite and heal the cracks. Any existing self-healing approach occurs only for one time. Intrinsic self-healing only is effective for ultrafine cracks.
The conventional approaches involve extrinsic and intrinsic healing mechanism. In extrinsic healing, hollow fibers or capsules filled with healing agent and catalysts are encapsulated in polymer composite materials. The capsules rupture on damage of polymer composite, and healing agent is delivered, which fills the crack and is cured in presence of catalyst and heals the composite materials. The major issue with the approach is that it works only once and cannot heal any further damage after the capsules or hollow fibers are fractured and the curing agent is used.
Intrinsic healing involves extensive chemical modification of polymer chains of polymer composite which completely change various properties such as adhesion and mechanical properties, etc. Intrinsic self-healing coatings are not able to heal large cracks, which is a major drawback.